In recent years, due to an increase in the speed of communication, optical interconnects are becoming popular by which two or more semiconductor devices (e.g., Central Processing Units [CPUs]) are connected to each other with an optical fiber, which achieves a higher speed and a higher level of performance than related metal wirings. In an optical interconnect structure, an electric signal that is output from a semiconductor device is first converted into an optical signal by an optical module and is subsequently output to an optical fiber. To be compliant with high-speed communication, the distance allowance between the semiconductor device and the optical module is short, e.g., approximately 10 cm. Thus, on a substrate, the optical module is disposed to be positioned either near or on the semiconductor device, which is a heat generating member. In addition, because the optical module itself generates heat due to the light emission, it is desirable that the optical module has excellent heat radiation characteristics.
Patent Document 1: Japanese Laid-open Patent Publication No. 2013-050484
However, when the optical module is installed on the substrate, the optical fiber is connected to the surface opposite to the installment surface. As for the semiconductor device, unlike the optical module, it is possible to radiate heat therefrom by installing a heat sink in an upper part thereof. However, as for the optical module, the optical fiber hinders installation of a heat sink. Accordingly, it is difficult to improve the heat radiation characteristics of the optical module. In particular, when a light emitting element included in the optical module is a Vertical Cavity Surface Emitting Laser (VCSEL) element or the like, the optical module has a structure in which a resonator is disposed in the thickness direction and therefore emits light in the vertical direction (i.e., the direction perpendicular to the horizontal plane), unlike the situation where the light emitting element is a Distributed Feed Back (DFB) laser element or the like. Thus, the optical fiber is connected to the upper surface of the optical module, and not to a lateral surface thereof. It is therefore difficult to radiate heat by installing a heat sink.
Further, VCSELs used as light emitting elements in optical modules are configured with GaAs crystals. Because a resonator is provided as described above, a VCSEL has multi-layer films formed above and below a light emitting unit (an active layer), the multi-layer films being configured to vary in refractive indexes by mixing other chemical elements with GaAs. Because the thermal conductivities of the multi-layer films are lower than that of the active layer, it is difficult for the heat generated at the light emitting unit to escape to the outside of the substrate and the like, and the heat therefore is locally present at the light emitting unit. Further, as for the optical fiber, because an optical waveguide (a core and a cladding) is formed by using SiO2, and also, coating is formed by using resin, the thermal conductivity of the optical fiber is low. Consequently, the heat that is locally present at the light emitting unit does not get radiated through the optical fiber which is positioned immediately above the light emitting element having the light emitting unit, either. An increase in the temperature of the light emitting unit can be a cause of degradation of the properties and the reliability of the light emitting element, and consequently, a cause of degradation of those of the optical module.